Post-fledging ecology of two songbird species across a rural-to-urban landscape gradient
Ian J. Ausprey, MS
Advisor: Amanda D. Rodewald
Urbanization alters the composition and structure of bird communities, yet little is known about the demographic processes underlying these patterns. Among the well-described urban ecological phenomena that could affect avian demography are increased abundances of generalist predators and invasive exotic shrubs. Such urban-associated changes should have particularly strong demographic consequences during the post-fledging stage of the avian life cycle, when juvenile birds have limited flight capabilities, are dependent upon parents for resource acquisition, and are vulnerable to predation. While ecologists have assumed that survival rates of fledgling birds are depressed in urban landscapes, few studies have tested this assumption. To understand how urbanization influences the ecology of post-fledging birds, I asked four broad questions: 1) How does fledgling survivorship vary across an urban-to-rural landscape gradient? 2) To what extent is variation in survivorship explained by fledgling age, energetic condition at time of fledging, and habitat selection? 3) How does the presence of Amur Honeysuckle (Lonicera maackii), an abundant exotic shrub, influence fledgling survivorship and habitat selection? and 4) Which ecological factors explain variation in natal home range extent and post-fledging dispersal timing for fledgling songbirds within an urbanizing landscape? From April - August 2008 and 2009 I used radio telemetry technology to track the fate and movements of fledgling Northern Cardinals (Cardinalis cardinalis) (n = 45) and Acadian Flycatchers (Empidonax virescens) (n = 31) living in a network of riparian forests distributed along a rural-to-urban landscape gradient in central Ohio, USA. The two species respond differently to urbanization; cardinals are "urban adapters" because they become more abundant in urban forests, and flycatchers are "urban avoiders" because they become less abundant as landscapes urbanize. Like other studies, I found that predation was the primary cause of mortality of post-fledging birds, and survivorship was lowest during the first few days after fledging. Curiously, cumulative survivorship (+ SE) of the urban avoider flycatcher was higher (0.720 + 0.097; 22 days) than that of the urban adaptor cardinal (0.440 + 0.077; 71 days). Across the entire post-fledging period, survivorship was not influenced by urbanization for either species. However, during the initial three days post-fledging when mortality was highest, survivorship of cardinals was promoted by an urbanizing landscape matrix. Cardinals and, to a lesser extent, flycatchers selected microhabitats that were more structurally complex than those at random plots or nest sites. In particular, cardinal fledglings selected areas rich with honeysuckle and saplings. While survival was not associated with cover by honeysuckle specifically, survivorship of both species improved with increasing structural complexity of the understory and midstory forest strata. Habitat structure also seemed to influence natal home range size (+ SE) in Acadian Flycatchers (1.91 + 0.24 ha), which had larger ranges in areas with more honeysuckle cover, saplings and mature trees. In contrast, variation in cardinal natal home range size (0.93 + 0.13) was not well explained by a suite of physiological, social and habitat variables. Timing of dispersal of cardinals (46 + 2 days) was best explained by and positively related to territory density of conspecifics.
Collectively, my results indicate that a variety of ecological factors influence the survivorship and movements of fledglings in urban landscapes. Predation and habitat selection play important roles in regulating fledgling survivorship, especially during the first few days out of the nest. Habitat structure also appears to partially explain variation in home range size for flycatchers. The fact that urbanization did not negatively influence fledgling survivorship suggests that in spite of abundant predator communities, urban forests may be capable of providing suitable habitat to juvenile birds. In a rapidly urbanizing world, land use planners should strongly consider the role urban forests play in sustaining bird populations when identifying conservation priorities.